In recent years, the advent of media which provide high vision images and multi-channel audio, such as digital broadcasting and blu-ray discs, has allowed home users to easily enjoy content with a high-quality sound and image. Furthermore, along with the widespread use of thin large-screen televisions, the home theater system for home movie entertainment has come under the spotlight. An audio reproduction system is especially in demand, which can provide a reproduced sound with high audio quality and high realistic sensation commensurate with the large-screen. Blu-ray discs, in particular, have a format with which 13.1-channel audio signals can be recorded (existing content is for 7.1 channels at a maximum), a sound field with high realistic sensation is expected to be provided by reproduced sounds of the blu-ray discs. Meanwhile, with the increase of the number of the channels, the number of speakers necessary for the reproduction increases as well as the realistic sensation, resulting in a step in the opposite direction to the easiness of the home theater system.
In view of the above, a multi-channel sound field reproduction apparatus has been conventionally proposed which has a multi-channel sound field reproduction system for reproducing 7.1-channel signals using 5.1-channel speakers.
FIG. 1 is a diagram illustrating an example of a conventional multi-channel sound field reproduction apparatus (see, Patent Reference 1).
The following describes an operation performed by the conventional multi-channel sound field reproduction apparatus with reference to FIG. 1.
FIG. 1 shows an example of a multi-channel sound field reproduction system for reproducing 7.1-channel signals using 5.1-channel speakers. The multi-channel sound field reproduction system includes: an arithmetic unit F1a which generates a sum signal and a difference signal of back surround signals BL and BR; a finite impulse response (FIR) filter F1b which processes the sum signal; a FIR filter F1c which processes the difference signal; an arithmetic unit F1d which generates a sum signal and a difference signal of signals processed by the FIR filters F1b and F1c; and an adder a and an adder b which add the sum signal and the difference signal processed by the arithmetic unit F1d to a side surround signal SL and a side surround signal SR, respectively.
Speaker placement according to the ITU-RBS. 775-1 recommendation, as shown in FIG. 2, is suggested for audio reproduction with the 5.1-channel system. Back surround channels which are added in the 7.1-channel system are placed further backward than SL and SR, at approximately 150 degrees. With the example of the conventional technique, the audio signals BL and BR of the back surround channels are processed and added to the surround channel signals SL and SR, thereby producing an effect of the 7.1-channel speakers with a 5.1-channel speaker configuration in which speakers for back surround channels are not provided.
First, 5.1-channel surround speakers SR and SL are placed not in the direction of 120 degrees; that is, diagonally backward right and diagonally backward left with respect to a viewer, but in the direction of 90 degrees; that is, in the direction immediately right and in the direction immediately left with respect to the viewer.
In processing the back surround signals, a pair of back surround signals BL and BR are calculated in the arithmetic unit F1a to generate a sum and a difference components. Then the sum signal is processed by the FIR filter F1b, the difference signal is processed by the FIR filter F1c, and the arithmetic unit F1d generates a sum and a difference signals. Transmission characteristics P and N of the FIR filters F1b and F1c are represented by the expressions below.P=(F+K)/(S+A)N=(F−K)/(S−A)Here, S indicates the transmission characteristic from a real speaker to the ear of the viewer on the same side as the real speaker, A indicates the transmission characteristic to the opposite ear of the viewer, F indicates the transmission characteristic from a position at which a sound image is to be localized to the ear of the viewer on the same side as the position, K indicates the transmission characteristic from the position at which a sound image is to be localized to the opposite ear of the viewer, and a head-related transfer function of the viewer is used.
As described above, a pair of back surround signals BL and BR are subject to sound image localization processing, added to audio signals of the SL channel and the SR channel, respectively, by the adder a and adder b, supplied, as output signals of the SL channel and the SR channel, to left and right side surround speakers SL and SR to be reproduced. It is possible, as described above, to realize sound image localization and realistic sensation of 7.1 channels easily at home with the 5.1-channel speaker configuration, by performing sound image localization processing on back surround signals to be added to the side surround speakers and reproduced.